<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <!-- 将div#demo.demo转化成<div id="demo" class="demo"></div> -->
    <script>
        const str = 'div#demo.demo';
        // str匹配正则
        const reg = /^(\w+)#(\w+).(\w+)$/;
        //替换指定部分
        const tag = str.replace(reg, function(match, $1, $2, $3) {
            return `<${$1} id="${$2}" class="${$3}"></${$1}>`
        })
        console.log(tag);
    </script>
</body>

</html>